Graphitized edm wire

Abstract

An electrode wire for use in an electrical discharge machining apparatus includes a core having a surface and one of a metal, an alloy of a metal, and a combination of a metal and alloy of a metal. An adherent coating of graphite is metallurgically bonded to the surface of the core.

Description

RELATED APPLICATIONS[0001]This application claims priority to U.S. Provisional Application No. 61 / 366,963, filed Jul. 23, 2010; U.S. Provisional Application No. 61 / 298,706, filed Jan. 27, 2010; and U.S. Provisional Application No. 61 / 496,639, filed Jun. 14, 2011, the entirety of which are incorporated herein by reference.TECHNICAL FIELD[0002]The present invention relates to electrical discharge machining (EDM) and, more specifically, relates to an electrode wire to be used in discharge machining and to the process for manufacturing an EDM electrode wire having a layer that includes graphite metallurgically bonded to the wire core.BACKGROUND[0003]The process of electrical discharge machining (EDM) is well known. In the field of traveling wire EDM, an electrical potential, i.e., voltage, is established between a continuously moving EDM wire electrode and an electrically conductive workpiece. The potential is raised to a level at which a discharge is created between the EDM wire electr...

AI Technical Summary

Benefits of technology

The patent describes a modified slip casting procedure for bonding an EDM wire to a graphite coating. A slurry of zinc powder, organic binder, colloidal graphite, and a suspension medium is used to coat the wire. The wire is then heat treated to remove the suspension medium and cure the binder, resulting in a metallurgical bond between the wire and the graphite coating. Another embodiment involves using a powder graphite lubricant to facilitate the adherence of powder particles to the wire and the formation of microscopic chemical bonds between the wire surface and the graphite coating. The resulting graphite layer is electrically conductive and adherent to the wire core. Overall, the patent provides techniques for improving the bonding quality and performance of EDM wires.

Problems solved by technology

It is important for flushing to be efficient because inefficient flushing result in conductive particles being built up in the gap, which can create the potential for electrical arcs.

Arcs are very undesirable as they cause the transfer of a large amount of energy, which causes large gouges or craters, i.e., metallurgical flaws, to be introduced into the workpiece and the EDM wire electrode.

Such flaws in the wire could cause the EDM wire to break catastrophically.

Fracture toughness is a measure of the resistance of a material to flaws which may be introduced into the material and that can potentially grow to a critical size to potentially cause catastrophic failure of the material.

Others in the prior art, for instance U.S. Pat. No. 5,762,726, recognized that the higher zinc content phases in the copper-zinc system, specifically gamma phase, would be more desirable for EDM wire electrodes, but the inability to cope with the brittleness of these phases limits the commercial feasibility of manufacturing such wire.

However, epsilon phase was found to be too unstable to be incorporated in the resultant high zinc alloy coating, although the potential for brittle epsilon coatings was acknowledged.

The challenge of incorporating graphite into other EDM wire systems is that of figuring out how to adhere the graphite to the wire.

This approach, however, was only marginally successful for an obvious reason.

In the case of a dense copper oxide coating, the oxide may well be adherent, but the lack of porosity requires the graphite coating to lay on top of the oxide and therefore it cannot be captured by the oxide and is subject to being “peeled” off the surface.

Other inventors, for example those listed in U.S. Pat. No. 5,030,818, have suggested incorporating unrealistically large volume fractions of graphite, e.g., up to 40 weight percent graphite, into a molten bath of copper or brass and “solidify(ing) into wire with a diameter of about 0.002 to about 0.014 inches.” Such a large volume of graphite, however, could never be uniformly dispersed in a molten bath of copper or brass due to the large density difference between graphite and metals.

Even if the graphite could be uniformly incorporated into the melt, the suggestion that wires of diameters ranging between 0.002 and 0.014 inches could be continuously cast is unrealistic.

This method of applying graphite to an epsilon phase brass coating, however, is unsuccessful because no adhesion is formed between the graphite and the epsilon phase brass and, thus, the graphite coating is not adequately secured to the coating.

It has been suggested that other hard inert particles incorporated into coatings will enhance the erosion resistance of the coatings, but one would not expect graphite to function similarly because it is not known to have significant erosion resistance.

The issue with both hard inert particles and graphite is adequately adhering the discrete particles on and into the coating.

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